The field of the invention is esters and processes of making the same from polycarboxylic acids. The present application is particularly concerned with the preparation of dimethyl terephthalate by the esterification of terephthalic acid.
The state of the art of preparing dimethyl terephthalate may be ascertained by reference to the Kirk-Othmer "Encyclopedia of Chemical Technology", 2nd Ed., Vol. 15 (1968), pages 466-467, under the section "Current Commercial Processes for Polymer-Grade Dimethyl Terephthalate", and by reference to U.S. Pat. No. 2,876,252 of Rudolf Lotz et al., which issued Mar. 3, 1959; U.S. Pat. No. 3,617,226 of Ferdinand List et al., which issued Nov. 2, 1971; and U.S. Pat. No. 3,364,251 of Anton Benning et al. which issued Jan. 16, 1968; British Patent No. 1,053,164 published Dec. 30, 1966; German Published Application No. 1,667,430 corresponding to French Patent No. 1,585,305; and German Published Application No. 1,933,946 of Ferdinand List and Helmut Alfs, published Jan. 21, 1971, the disclosures of which are incorporated herein.
The state of the art of fluidized beds, particularly for use in catalysis and gas-solid reactions, may be ascertained by reference to Kirk-Othmer ibid, Vol. 9 (1966), under the section "Fluidization", pages 398-445.
The present invention is particularly concerned with a process for the continuous esterification of terephthalic acid with methanol in the gas phase in a system consisting of a pre-reactor and a connecting solid bed catalyzer.
The use of terephthalic acid as the basic material for the production of polyester fibers is well known. Since the acid, in the usual commercial production process is not produced as purely as is required for the polycondensation reaction, purification methods must be added to the production which can effectively remove the unfavorable physical properties of commercial terephthalic acid. In most cases, the purification of the acid is undertaken in the form of its dimethyl ester which can be introduced with the regeneration of methanol to the interchange of ester radicals and the preliminary condensation with glycol or other diols.
Therefore, there is a considerable interest in an efficient process for esterifying terephthalic acid. Methanol, because of its light molecular weight and its low cost, is the most favorable alcohol component, although the esterification with methanol poses special technical problems of a chemical nature. Terephthalic acid, itself is practically insoluble in boiling methanol, and the esterification can be executed with sufficient speed only in pressure resistant apparatus made of high alloy steel.
Therefore, a process has been developed whereby terephthalic acid is esterified in the gas phase with superheated methanol vapor, in part by adding inert gas as a carrier gas. This process is disclosed in U.S. Pat. No. 2,876,252, whereby pulverulent terephthalic acid is reacted with methanol at approximately 300.degree.C, by spraying catalytic mixtures of the acid and methanol into a reaction tube, to yield relatively little output of a dimethyl terephthalate of little purity. According to U.S. Pat. No. 3,364,251, methanol, and if necessary with nitrogen added, is fed upwards into a fluidized bed of catalysts and pulverulent terephthalic acid, and the dimethyl ester, quickly formed at the high reaction temperatures of approximately 300.degree.C, is discharged in the form of gas, together with the excess methanol and the water of reaction. The dynamic conditions of the fluidized bed are, however, disadvantageous for general use: the particle size distribution of the injected terephthalic acid and the whirling conduct of the acid and of the esterification catalyst can bring about an undesired channel formation and must be adjusted to each other within a relatively narrow margin, to prevent one of the two components or parts thereof from being discharged earlier from the reactor.
British Patent No. 1,053,164 demonstrates the esterification of terephthalic acid after vaporization in the superheated methanol vapor stream in contact with a solid catalyst. Thereby, pelleted terephthalic acid becomes vaporized from a 315.degree. to 345.degree.C heated sublimation zone with methanol of approximately 400.degree.C, and is esterified in a follow-up reactor at approximately 300.degree.C. For the process of heating and vaporizing the terephthalic acid that is brought into the sublimator, the thermal conductivity of the superheated methanol can be used. This, in itself simple, process of esterification requires a tenfold amount by weight of methanol at 400.degree.C and also a continuously operating sublimator, in order to heat the terephthalic acid of approximately 20.degree.C to an average sublimation temperature of 330.degree.C and to completely vaporize it at this temperature.
On the other hand, in the process according to U.S. Pat. No. 3,617,226, whereby pulverulent terephthalic acid is mechanically mixed with a suitable solid catalyst in a horizontal cylindrical container and exposed to methanol in the gaseous phase from 300.degree. to 320.degree.C, the particle size of the esterification catalyst and acid need not be correlated, and the resulting terephthalic acid dimethyl ester leaves the reactor in the gaseous state. Since, at these reaction temperatures, the esterification speed is extremely high, the transformation of the terephthalic acid and volume-time output become dependent upon technical process factors such as velocity of diffusion, the ratio of the catalytically active surface to the terephthalic acid, transportation of material and heat, dwell, et al. Dwell and transformation are, in the first approach, dependent upon the catalyst surface available for the time unit and the amount of heat which is needed to vaporize the terephthalic acid and the dimethyl terephthalate, respectively. In this case, the catalyst surface, available per unit of time, is determined by the useful volume of the cylindrical vessel, while the transferable amount of heat, apart from the possible temperature difference between the heating medium and the reactants, is limited by its surface. By enlarging these cylindrical reactors, however, the ratio of the surface to the volume becomes smaller, so that for great esterification capacities a limitation occurs because of the technical possibilities of producing cylindrical reactors with the highest possible heat surfaces and stirring devices.